Recently, various wireless communication systems, such as mobile phone systems, systems using wireless LAN, and communication systems using ZigBee (registered trademark) technology, are proliferating. In such wireless communication systems, many devices are disposed, with each device communicating individually. For this reason, in such wireless communication systems, multiple devices may simultaneously transmit data packets in some cases. In such cases, there is a risk that data packets may collide and data may become unreadable.
Thus, in such wireless communication systems, CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is implemented to decrease data packet collisions even in cases where multiple devices each transmit a data packet. With CSMA/CA, a device that wants to transmit a data packet senses communication conditions on a channel (this may be hereinafter referred to as “carrier sense”), and initiates data packet transmission if the channel is clear.
Herein, if a given device is taken to be a device A, a device whose transmitted signal does not directly reach the device A is designated a “hidden node” with respect to the device A. Specifically, a hidden node is a device whose signals do not reach because of a wall or other obstruction between it and the device A, or because of a long distance to the device A. In other words, a device may be unable to sense the communication conditions of a hidden node with respect to that device. In contrast, if a given device is taken to be a device A, a device whose transmitted signal directly reaches the device A, or in other words, a device other than a hidden node excepting the device A, is designated an “exposed node” with respect to the device A. In other words, a device may be able to sense the communication conditions of an exposed node with respect to that device.
In the related art, there has been proposed technology that sets a suitable value for the number of data packet retransmit attempts and technology that computes and evaluates communication quality in such wireless communication systems implementing CSMA/CA.
For example, there is technology of the related art that specifies a fixed number of retransmission attempts in advance for all devices in a wireless communication system implementing CSMA/CA. With this technology of the related art, a system designer predicts or measures an average packet error rate (PER) in advance, computes an optimal number of retransmission attempts from the PER, and sets that number of retransmission attempts for each node. Additionally, a report is issued to a base station if the number of retransmission attempts exceeds a set value. The base station may stand by for a fixed amount of time, or issue a report to a system administrator.
As another example, there is technology of the related art that evaluates communication quality using a receive signal strength indication (RSSI) and shortens the retransmit interval when the RSSI value lowers in a wireless communication system implementing CSMA/CA. Communication quality is considered to be poor when the RSSI value is low. In such cases, it may take time for a device to complete transmission of a data packet if the retransmit interval is long. Thus, in this technology of the related art, such situations are avoided by shortening the retransmit interval.
Such technology of the related art is disclosed in Japanese Laid-open Patent Publication No. 2007-174292, Japanese Laid-open Patent Publication No. 2010-103815, and Japanese Laid-open Patent Publication No. 2006-13841, for example.